The invention relates to a method and a radio communication system for transmitting speech information.
The cellular mobile radio system GSM (Global System for Mobile Communication) is an example of a radio communication system. In this system, the multiple access method TDMA (Time Division Multiple Access) is used, whereby mobile stations are distinguished by different time slots of a time division multiplex system.
FIG. 1 depicts a block diagram of such a mobile radio network whose service area is divided into a plurality of cells Z. In a cell Z, the telecommunication service from and to a mobile station MS is made available by a base station BTS. One or more base stations BTS are connected to a base station control BSC. The base station control BSC executes the local functions of the call switching, monitoring, and maintenance. The adaptation of the data of the mobile radio network to a telephone network is performed by a transcoder unit TRAU.
The totality of the base stations BTS, the base station control BSC, and the transcoder unit TRAU is referenced as a base station system BSS. A plurality of base station controls BSC are connected to an exchange MSC, which is a constituent of a switching means SSS which is connected to a public telephone network PSTN. The telephone network PSTN can be constructed as an ISDN network, as another mobile radio network, or as another kind of telephone or data network. The interface between base station BTS and base station control BSC is referenced as an Abis interface, and the interface between base station control BSC and transcoder unit TRAU is referenced as an Asub interface. The transmission occurs in wire-bound fashion via a PCM24 or a PCM30 link.
In a transmission of speech to or from a mobile station, it is generally known to sample analog speech information on the transmit side and to convert this into digitally coded speech information in a speech coder, and to transmit this error-protected, as warranted. On the receive side, the digitally coded speech information is converted back into analog speech information.
In the above-mentioned mobile radio network GSM, which serves below only to illustrate the technical background of the present invention without restricting the generality of its use, the transcoder unit TRAU executes the adjustment of the data rate which is communicated by the telephone network PSTN to the data rate specified in the mobile radio network, (see German reference DE 196 47 630). A speech coding is executed not only in the mobile station MS, but also in the transcoder unit TRAU. A channel coding is typically executed for a radio transmission between mobile station MS and base station BTS.
With the introduction of radio communication systems of the third generation (UMTS), it is provided that speech information should be transmittable broadband beyond the bandwidths from 300 Hz to 3.4 kHz, which are typical in telephone technology. The previously employed narrowband transmission should thus be replaced, and the speech quality improved. In previous narrowband transmission methods, it is taught in the reference xe2x80x9cMore Natural Sounding Voice Quality Over the Telephonexe2x80x9d (M. Abe and Y. Yoshide, NTT Review, Vol. 7, Nr. 3, May 1995:104-109) to execute an artificial bandwidth expansion on the receive side, in order to imitate a natural speech sound. This described bandwidth expansion is applied continuously and should be independent of the conditions of the terminal equipment (see p. 105, bottom, to p. 106, top).
It is an object of the present invention to provide a method and radio communication system for transmitting speech information which can correspond to the variable transmission conditions with respect to the speech coding/decoding.
The inventive method for transmitting speech information in a radio communication system presupposes speech coder/decoders, which are arranged in a mobile station, or respectively, in a transcoder unit. By means of the coder/decoders at least one broadband and one narrowband speech coding method can be selected. Furthermore, for a connection to the mobile station, a monitoring of transmission possibilities is executed, and given limited transmission possibilities, a switchover is executed from the broadband to the narrowband speech coding method. That is, the speech information is transmitted narrowband via at least parts of a transmission channel between the speech coder/decoders. The broadband speech coding method requires a high net data rate, which no longer exists with sufficient error protection under these transmission conditions.
A narrowband speech coding method leads to a different speech perception at the receive side, however, whereby the changeover, in particular, leads to undesirable and disruptive sound alterations. The received narrowband speech information is inventively expanded to a greater bandwidth at the receive side. The subjective speech impression improves by means of the bridging of this changeover effect. This applies particularly to short changeover phases from broadband to narrowband speech coding methods.
According to an advantageous development of the present invention, the selected narrowband speech coding method provides a higher coding protection than the broadband speech coding method. Thus, in the case of temporary signal interruptions, which are frequent in radio communication systems, the transmission is guaranteed, and the previously perceived subjective speech impression is maintained at the listener.
According to other advantageous embodiments of the present invention, there are reactions to limitations of the transmission possibilities, which limitations are brought about, via the radio interface, by an at least partial failure of a previously used tandem free operation (TFO) mode or by poor transmission conditions. The limited transmission possibilities can also arise due to a changeover to another data rate (for example, half-rate), which is necessary due to the capacity utilization of system resources. In all these cases, the inventive method creates an improved subjective speech impression.
The speech coding for the connection is advantageously executed in a TFO mode, and the changeover is thus executed for both devices taking part in the connection, whether it be a mobile station, another terminal equipment, or a transcoder unit. The TFO mode replaces a speech coding/decoding at the network side, in the transcoder unit TRAU, for example, by a transparent transmission of the speech-encoded speech information from terminal equipment to terminal equipment. Due to the transparency of the TFO mode, given transmission disturbances, an adaptation must also occur for both transmissions. However, it is just as possible that a failure of the TFO module represents the disturbance and that there must be a changeover to a PCM transmission at the network side. The inventive method leads to an improved speech impression in the changeover phase in this case also.
Another frequently occurring disturbing influence is a handover process during the TFO mode. The TFO mode is then potentially temporarily interrupted, and there is a changeover to a narrowband speech coding until the TFO mode is reestablished.
If the interference lasts longer, for example, longer than 5 s, then it is advantageous to gradually reduce the bandwidth expansion of the received narrowband speech information. The transition is thus smoothed for a subsequent narrowband transmission. The bandwidth expansion can be gradually reduced or can be suppressed by digital filtering. These adjustments can be executed according to the individual receiver.
According to an advantageous embodiment of the present invention, the speech coder/decoder is subdivided into modules for at least two frequency ranges, so that the broadband, or respectively, narrowband speech coding method can be specified by the selection of one or more modules. These modules are easy to add, or respectively, remove, so that the speech coder/decoder can be adapted to the transmission conditions by simple parametrization.
It also is within the framework of the present invention that the speech coder/decoder provides two broadband speech coding methods (i.e., the bandwidth is greater than 300 Hz to 3.4 kHz), the more narrowband of the two speech coding methods being more sharply limited in the higher frequency range. But an addition of the lower frequencies already improves the sound perception. This also has the advantage that e.g. 50 Hz to 3.4 kHz can be transmitted PCM-encoded. The higher frequencies are additionally artificially generated in a bandwidth expansion.